Gain reflector-liquid crystal display

ABSTRACT

A display apparatus comprising a display medium and a gain reflector disposed behind the display medium for reflecting incident light. The display medium may comprise a liquid crystal material containing a dye that conforms to the structure of the liquid crystal material and a containment medium for inducing distorted alignment of the liquid crystal material which in response to such alignment scatters and absorbs light and which response to a prescribed input reduces the amount of such scattering and absorption.

This is a continuation of application Ser. No. 147,756 filed 1/25/88 nowU.S. Pat. No. 4,991,940.

BACKGROUND OF THE INVENTION

The present invention relates generally to displays, and moreparticularly to displays utilizing a gain reflector and a display mediumthat may be switched between light scattering and non-scattering states.

Visual display devices may utilize liquid crystals. The property ofliquid crystals that makes them particularly useful in visual displaysof the type of the present invention is the ability of certain liquidcrystals materials to transmit light in a strictly aligned ornon-scattering state, and to scatter light and/or to absorb itespecially when combined with an appropriate dye, in a relatively freeor scattering state. An electric field may be selectively applied acrossthe liquid crystals to switch between scattering an non-scatteringstates.

It is desirable that liquid crystal visual displays have excellentcontrast between the characters displayed and the background, and highbrightness in all ambient light conditions. It is also desirable thatthe display be free of front surface glare.

The present invention relates in a preferred embodiment describedhereinafter to the use of a liquid crystal as a display medium that maybe designated encapsulated operationally nematic liquid crystal materialor nematic curvilinearly aligned phases ("NCAP") liquid crystalmaterial.

A detailed explanation of operationally nematic or NCAP liquid crystalmaterial is provided in U.S. Pat. No. 4,616,903 issued Oct. 14, 1986, inthe name of Fergason, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD,assigned to Manchester R&D Partnership, the disclosure of which ishereby incorporated by reference. Reference may also be made to U.S.Pat. No. 4,435,047, issued Mar. 6, 1984, in the name of Fergason,entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to ManchesterR&D Partnership, which disclosure is also hereby incorporated byreference.

In the field-off condition, or any other condition which results in theliquid crystal being in a distorted or randomly aligned state, the NCAPliquid crystal material scatters incident light. In the field-oncondition, incident light is transmitted through the NCAP material.

A pleochroic dye may be present with the liquid crystal material toprovide substantial attenuation by absorption in the field-off state butto be substantially transparent in the field-on state. Any reference tothe ability of liquid crystal to scatter and/or absorb light inaccordance with the present invention should not be limited to thescattering and minimal absorption properties of liquid crystal butshould include the additional properties pleochroic dyes may impose onthe optical properties of the liquid crystal.

The display medium of the display of the present invention may alsocomprise other scattering-type display materials, e.g., dynamicscattering liquid crystal systems or ferroelectric ceramic systemscomprising optically clear (Pb,La)(Zr,Ti)(O₃) ("PLZT"). The dynamicscattering and PLZT display mediums are both switchable between lightscattering and non-scattering states.

In reflective liquid crystal displays utilized heretofore, the use of again reflector can produce higher brightness in the field-on state whenthe illumination on the display is collimated or quasi-collimated.However, in the field-off state, brightness is also increased, therebyproviding little or no improvement in the contrast ratio. When theillumination on such displays is diffuse, the gain reflector will notaffect the brightness at all.

The present invention relates to improvements in reflective displaysutilizing a display medium variable between light scattering andnon-scattering states. The present invention also relates to the use ofthe light scattering and absorption characteristics of NCAP liquidcrystal materials. The invention further relates to the use of suchliquid crystal materials and characteristics, together with a pleochroicor diochroic dye, for example, to obtain relatively high contrast anddark characters or information displayed on a relatively brightbackground in both small and large size displays.

An object of the present invention is to provide a display having arelatively high contrast as well as brightness.

A further object of the present invention is to provide a display thathas excellent contrast and high brightness in all ambient lightconditions.

Another object of the present invention is to improve the performance ofa display in viewing conditions where glare is present.

SUMMARY OF THE INVENTION

As may be seen hereinafter, the display disclosed herein is one whichcomprises a display medium disposed at a viewing side of the display.The display medium is switchable between a first state in which incidentlight is at least one of scattered and absorbed, and a second state inwhich the amount of such scattering or absorption is reduced. A gainreflector means for reflecting light transmitted by the display mediumis located behind the display medium.

The display medium may comprise a liquid crystal material containing adye that conforms to the structure of the liquid crystal material and acontainment medium means. The containment medium means induces adistorted alignment of the liquid crystal material which in response tosuch alignment scatters and absorbs light and which in response to aprescribed input induces the amount of such scattering and absorption.

The gain reflector means may be an offset gain reflector that providesthat specular reflection or glare is angularly offset from the reflectedgain, i.e., light reflected by the offset gain reflector. The displaymay also include a color filter or lens disposed between liquid crystalmeans and the gain reflector means.

In accordance with one aspect of the present invention, a liquid crystaldisplay can produce relatively bright or white characters, information,etc. on a relatively dark background in collimated, quasi-collimated ordiffuse lighting conditions. The dark background may be produced byliquid crystal material that is randomly aligned in the field-off statewherein light incident on the liquid crystal material is scattered andabsorbed. The bright characters are caused, for example, by liquidcrystal material that is in a field-on state or in ordered alignment andthus, substantially optically transparent. When the liquid crystalmaterial is in the field-off state, only the relatively dark backgroundappears. When a selected portion of the liquid crystal material is inorder alignment, the field-on state, a very bright character will appearagainst the dark background to an observer within a viewing angle of thedisplay.

BRIEF DESCRIPTION OF THE DRAWINGS

The display of the present invention will be described in more detailhereinafter in conjunction with the drawings wherein:

FIG. 1 is a schematic, side elevational view illustrating a displayapparatus in accordance with the present invention;

FIG. 2 is a schematic view illustrating the gain reflector component ofthe display apparatus of the present invention;

FIGS. 3 and 4 are schematic illustrations of a liquid crystal materialused in the invention including a volume of liquid crystal with a dye ina containment medium means with the liquid crystal structure indistorted and parallel alignment, respectively;

FIG. 5 is a schematic view illustrating an offset gain reflector thatmay be utilized in the display apparatus of the present invention;

FIG. 6 schematically and perspectively illustrates a form of an offsetgain reflector that may be utilized in the present invention;

FIG. 7 is a view along line 7--7 of FIG. 6;

FIG. 8 graphically and schematically illustrates a reflected lightpattern from an offset gain reflector that may be utilized in thedisplay apparatus of the present invention; and

FIG. 9 is a schematic view illustrating another embodiment of thedisplay apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals for likecomponents are utilized throughout the drawings, attention is firstdirected to FIG. 1. FIG. 1 shows a liquid crystal display apparatusindicated generally by reference numeral 10.

The display 10 includes two main components: a display medium 12 and again reflector 14. Display medium 12 is at a viewing side 25 of display10. The gain reflector 14 is at a non-viewing side 27. A color filter 20may be located in between display medium 12 and gain reflector 14.

The display may further include a frame 16. The frame may comprise aplastic housing that provides environmental protection for the displaymedium and the gain reflector.

The display medium comprises a material that may be switched between alight scattering and a non-scattering state, e.g., a NCAP liquid crystalmaterial, a dynamic scattering liquid crystal material or aferroelectric ceramic such as PLZT, all of which are discussed in moredetail hereinafter. These materials as utilized in the present inventionproduce displays having better contrast ratios then heretofore possible.

When a gain reflector is utilized with standard twisted nematic andguest-host liquid crystals, both the field-on and field-off states areincreased in brightness. Although the brightness is increased, thecontrast ratio of the display remains the same. The perceived appearanceof such a display may be slightly better than without the gainreflector. In some cases, the appearance may be worse.

However, in the display of the present invention, a major differenceoccurs when the display is switched between the scattering andnon-scattering states. The effective gain of a gain reflector dependsupon the degree of collimation of the incident light. In the scatteringstate, light incident upon the reflector will be relatively diffuse. Thegain of the reflector for diffuse illumination will be close to unity.In the non-scattering state, light incident on the reflector may be muchmore collimated (depending upon the design of the illumination system)and therefore the effective gain will be greater than unity.

For example, displays of the present invention utilizing the NCAP liquidcrystal material, have been found to have effective gains of 2.2 in thenon-scattering state and 1.1 in the scattering state. Thus, thebrightness in the field-on state is increased by a factor of 2.2 and thecontrast ratio (contrast ratio=brightness (on) X gain/brightness (off) Xgain) is doubled for displays having an effective gain of 2.2 in thefield-on state and 1.1 in the field-off state.

Needless to say, higher brightness with higher contrast produces majorimprovements in display performance and appearance.

The display medium 12 of the present invention may comprise a liquidcrystal cell 21 that includes a liquid crystal material 22. The opticalcharacteristics of the liquid crystal material according to theinvention are a function of whether or not a prescribed input is appliedto the liquid crystal material. The prescribed input is preferably ofthe electromagnetic type and, more particularly, an electric field.

A schematic representation of a circuit 24 for selectively applying ornot an electric field to the liquid crystal material 22 is illustratedin FIG. 1. Such a circuit 24 may include an electric power supply 26,such as a battery, and a switch 28. The electric power supply mayalternatively be a source of alternating current. The circuit 24 isconnected by electrical leads 30, 31 to electrodes 32, 33 positioned onopposite sides or surfaces of liquid crystal material 22 of cell 21.

The electrodes 32, 33 are substantially optically-transparent, and maybe formed on optically-transparent substrates 52, 53, respectively.

Operationally, with switch 28 open, no electric field is applied to theliquid crystal material, which then is in the so-called de-energized(field-off) condition or mode, or scattering light state. With switch 28closed, an electric field is applied across the liquid crystal material,which then goes into the so-called energized (field-on) condition ormode, or non-scattering light state. The operational characteristics ofthe display will depend on the scattering or non-scattering condition ofthe liquid crystal material 22, as is described in further detail below.

The liquid crystal material 22 preferably is of the type (NCAP)disclosed in U.S. Pat. No. 4,435,047. As is represented schematically inFIG. 3, such liquid crystal material 22 preferably is formed ofoperationally nematic liquid crystal 40 in a plurality of volumes 42formed in or defined by a containment medium 44. The liquid crystal 40preferably is optically transparent, and the containment mediumpreferably also is optically transparent. In the embodiment illustrated,preferably the liquid crystal material 22 was mixed therewith a dye 46,for example a pleochroic or diochroic dye. However, a liquid crystalmaterial without a dye may be utilized to form the display medium.

Each volume 42 may be discrete or alternatively the liquid crystal 40may be contained in a containment medium, such as a polymer encapsulantthat tends to form a multitude of capsule-like environments containingthe liquid crystal material. The liquid crystal 40 may be more or lessconfined to an approximately spherical or otherwise curvilinear surfaceof a containment cavity. Such cavities, however, may be interconnected,for example, by one or more channels or passages. The liquid crystalwould preferably be in both the discrete volumes or cavities and in theinterconnecting passages. Thus, the internal volumes of respectivecapsules may be fluidly coupled via one or more interconnectingpassages. All of the aspects and features of the present inventionvis-a-vis individual unconnected capsules have been found to beapplicable to an arrangement of capsules that have one or moreinterconnecting passages.

The pleochroic dye 46 in the liquid crystal 40 will absorb some of thelight transmitted therethrough, and the degree of such absorption is afunction of whether or not an electric field is applied to the liquidcrystal material and of the magnitude of such field. Preferably suchabsorption in the field-on condition of the liquid crystal should bezero or as close to zero as possible to maximize transmission ofincident light.

The dye alignment follows the alignment of the liquid crystal 40, as isillustrated schematically in FIGS. 3 and 4, for example, and isexplained in further detail in the above-mentioned patent. Therefore,when the liquid crystal structure is in distorted alignment, the dyewill provide a relatively substantial amount of light absorption.However, when the liquid crystal 40 is in parallel alignment, e.g., likethat liquid crystal shown in FIG. 4, light absorption by the dye will beminimized. As the magnitude of electric field is increased or decreased,the amount of distortion of the liquid crystal material will vary, andthe amount of absorption by the dye also will correspondingly vary.

In field-on operation, i.e. the non-scattering state, as shown in FIG.4, the liquid crystal structure is considered to assume a generallyparallel alignment. Since the ordinary index of refraction of the liquidcrystal 40 in field-on condition is matched to that of the containmentmedium 44, the liquid crystal material 22 becomes essentially opticallytransparent and light incident thereon is not refracted at interfacesbetween the liquid crystal and containment medium. During such field-onoperation, incident light is transmitted through liquid crystal cell 21.

Field-off operation, i.e. the scattering state, of the display isdepicted in FIG. 3. Light which is incident on liquid crystal material22 is refracted, scattered and absorbed. Such scattering is effectedbecause the extraordinary index of refraction of the liquid crystal 40is different from the index of refraction of the containment medium 44.The light is absorbed by the dye 46.

The index of refraction of the liquid crystal varies depending onwhether an electric field is applied across the liquid crystal material.The index of refraction of containment medium 44 and the ordinary indexof refraction (the index when an electric field E is applied) of theliquid crystal 40 should be matched as much as possible when in thefield-on state to avoid scattering, thereby tending to maximize lighttransmission. However, when the liquid crystal is in the field-offstate, there will be a difference in the indices of refraction at theboundary of the liquid crystal 40 and the containment medium.

In the field-off state, the containment medium tends to distort thenatural liquid crystal structure to present to a great extent at theinterfaces of the liquid crystal and surfaces, the extraordinary indexof refraction (the index with no electric field E) characteristic of theliquid crystal; and such extraordinary index of refraction is differentfrom the index of refraction of the containment medium. Therefore, whenin such distorted alignment condition, there is a difference in theindices of refraction at the interface between the liquid crystal andcontainment medium, which causes refraction and, thus, scattering oflight incident thereon.

As long as the ordinary index of refraction of the liquid crystal iscloser to the index of refraction of the containment medium, than is theextraordinary index of refraction, a change in scattering will resultwhen going from the non-scattering (FIG. 4) to scattering (FIG. 3)states, and vice-versa.

In accordance with the present invention, electrode 33 may, for example,form a common electrode surface while the opposed electrode 32 comprisespatterned electrodes having multiple electrode portions that can beselectively energized to apply the electric field to selected portionsof the liquid crystal material. For instance, as is well known in theart, electrode 32 may be divided into seven electrically isolatedsegments, each of which may be selectively energized to display variousnumerical characters. Electrode 32 may also be configured to form a dotmatrix display comprising a plurality of dots or pixels arranged incolumn and rows. A row is enabled to accept display information inparallel via the column lines.

The liquid crystal material 22 including dye 46 may be prepared in theform of an emulsion of liquid crystal and containment medium which issubsequently dried or cured. Alternatively, as noted heretofore, theliquid crystal material may take the form of a plurality of individuallyformed capsules of liquid crystal in the containment medium.

In one embodiment, the containment medium is formed of a polyvinylalcohol (PVA). In another embodiment, the liquid crystal is dispersed orentrapped in a latex containment medium. In either embodiment,substrates 52, 53 of liquid crystal cell 12 may comprise a polyesterfilm, such as Mylar®, that has been precoated with a layer of indium tinoxide (ITO) to form the electrodes. Preferably, the film has beenprecoated with a 80 to 500 ohms per square layer of ITO. Materials otherthan ITO may be used to form the electrodes of the apparatus of thepresent invention.

Latex entrapped NCAP liquid crystal comprises the entrapment of liquidcrystal in a latex medium. The latex is a suspension of particles. Theparticles may be natural rubber or synthetic polymers or copolymers. Alatex medium is formed by drying a suspension of such particles. Afurther explanation of latex entrapped NCAP liquid crystal and methodsof making the same are provided in application U.S. Pat. No. 705,209,filed Feb. 25, 1985, in the name of Pearlman, entitled LATEX ENTRAPPEDNCAP LIQUID CRYSTAL COMPOSITION, METHOD AND APPARATUS, assigned to theassignee of the present invention, and which disclosure is herebyincorporated by reference.

In an alternative embodiment, the display medium 12 may comprise aliquid crystal cell that consists of a dynamic scattering liquid crystalmaterial. As in the case of the above-described encapsulatedoperationally nematic liquid crystal material, a dynamic scatteringliquid crystal material is switchable between light scattering andnon-scattering states. As contrasted with the operationally nematicliquid crystal material, an electric field passed through a dynamicscattering liquid crystal material disrupts the alignment of the liquidcrystal material such that light is scattered or refracted. However, inthe field-off state, the dynamic scattering liquid crystal material isoptically clear. Thus, the scattering effect in the dynamic scatteringliquid crystal material is obtained when no electric field is applied.

Dynamic scattering liquid crystal materials are well known in the art,and as such, they will not be described in any detail herein. Dynamicscattering is described in the following articles, both of which areherein incorporated by reference: J. L. Fergason, et al., "LiquidCrystals and their Applications", Electro-Technology, January, 1970,p.41; and E. H. Heilmeier, et al., "Dynamic scattering: A newelectro-optic effect in certain classes of nematic liquid crystals",Proc. IEEE, Vol. 56, p.1162, 1968.

Other types of liquid crystal materials that may be utilized as thedisplay medium include twisted nematic and super twist liquid crystalmaterials. These materials are also well known to those skilled in theart, and are not described in any detail herein.

In yet another embodiment, display medium 12 may comprise ascattering/non-scattering ferroelectric ceramic system. Ferroelectricdisplay systems are also well known in the art, and as such, they willnot be described in detail. They may comprise optically clear(Pb,La)(Zr,Ti)O₃ ceramic materials (PLZT). The PLZT ceramic, like theencapsulated operationally nematic liquid crystal material and thedynamic scattering liquid crystal material, is switchable between lightscattering and non-scattering states. The PLZT ferroelectric ceramic isdescribed in the following articles, both of which are hereinincorporated by reference: A. L. Dalisa, et al., "Convolution ScatteringModel for Ferroelectric Ceramics and other Display Media", Proc. IEEE,Vol. 61, n. 7, pp. 981-991, July 1973. G. H. Heartling, et al., "Recentimprovements in the optical and electro-optic properties of PLZTceramics", Ferroelectrics, Vol. 3, p. 269, 1972.

The color filter 20, described above, is utilized to provide a colordisplay. Filter 20 may be constructed from any transparent,non-scattering color material. For example, the color filter may beformed from colored glass or a dyed plastic material. The color filter,color sheet or lens 20 may be virtually any color, for example red,green, yellow, orange, etc.

Color filter 20 may be laminated to the front or backside of displaymedium 12. Preferably, however, the filter is laminated to the back ofthe display medium.

Alternatively, as illustrated in FIG. 1, color filter 20 may be spacedfrom the back of display medium 12 such that an air gap existstherebetween, as is represented by spacing "d₁ ". If the electrodes ofthe display medium, are configured to form pixels the spacing "d₁ "should be approximately less than 10% of the minor dimension of thepixels.

As discussed, the display medium 12 is switchable between anon-scattering (clear) state and a scattering (opaque) state. Thecolored material behind the display medium or portion thereof in thefield-on state is visible to an observer or an observing instrument 58on viewing side 25 of the display.

The color filter 20 may be eliminated, and instead gain reflector 14 maybe selectively screen printed with colored dyes, for example fluorescentdyes, as shown generally by reference numeral 54. The colored dyesprovide a colored pattern that can produce color for pixels in thedisplay. The fluorescent dye increases brightness due to its ability toabsorb light over a wide range of frequencies and then to emit thislight at a particular color.

As shown in FIG. 2, incident light, represented by light beams 60, whichis refracted when it passes through display medium 12, is reflected backfrom gain reflector 14 as light beams 62 that make up a gain lobe 64.The incident light is also reflected as glare, as will be explained inmore detail below, from the surface of display medium 12.

The reflected light 62 is not uniformly distributed but is concentratedto some degree. The limiting case of a gain reflector would be that of aplane mirror. In that case, all the light in a collimated incident lightbeam remains collimated in the reflected beam, which is propagating in adirection such that the angle of incidence equals the angle ofreflection. Depending upon the exact nature of the surface of the gainreflector, the light distribution in the reflected beam may be broad ornarrow. The gain of such a reflector may be defined as the ratio of thelight flux into a detector (with a fixed solid angle at a given angle tothe surface) from the gain reflector to that from a Lambertianreflector.

As the incident light beam becomes less collimated, the distribution ofreflected light from the gain reflector broadens and therefore the gaindecreases. The limiting case occurs when the incident light beam, orillumination from viewing side 25, for example, is diffuse orLambertian. This results in a Lambertian reflected light distribution,that is a gain of unity.

The gain reflector 14 may be any number of well known and readilyavailable gain reflectors that provide light reflection of incidentlight. The gain reflector, for example, may comprise a retro-reflectorwhere the reflected light is along the same path or line as the incidentlight beam.

More preferably, the gain reflector 14 provides that the reflected gainis along a path that is different from the incident light. Such a gainreflector, for example, is described in U.S. Pat. No. 4,456,336, issuedJune 26, 1984, the disclosure of which is hereby incorporated byreference. The gain reflector 14 may also comprise a lenticular surfacethat has a repeating, simple element, such as a spherical or cylindricalsection, that is embossed into flexible PVC that is coated with aluminumpigment paint or other reflective media.

The gain reflector may further comprise, as shown in FIG. 2, an opaque,plastic or metal substrate 50 having a reflective coating 55. Thecoating can comprise a thin layer of silver or aluminum, for example asputtered aluminum coating, that has a rough or uneven surface.

Another type of gain reflector that may be utilized in the display ofthe present invention is described in U.S. Pat. No. 4,241,980 (the "980patent"), issued Dec. 30, 1980, in the name of Mihalakis, et al.,entitled BEAM EMISSION CONTROL MEANS, assigned to William C. McGehon,the disclosure of which is hereby incorporated by reference. The gainreflector described in the '980 patent is commercially available fromProtolite Corporation, Palo Alto, Calif. It is sold under the trademarkMirror Image. This gain reflector and a method of its manufacture arealso described in a paper entitled: G. Mihalakis, "Large ScreenProjection Displays", Proc. SPIE, Vol 760, p. 29, 1987, which is hereinincorporated by reference.

This gain reflector comprises an array of optical elements that arejuxtaposed to form a matrix of rows and columns. These optical elementshave both convex and concave image-forming portions such that theoptical axes of the elements are at an angle to the normal to asubstrate of the gain reflector. The individual optical elementstypically have dimensions smaller than an observer can resolve at thedetermined viewing distance, and the convex and concave portions areshaped to provide overlapping images at that viewing distance.

As shown in FIG. 2, the glare or specular reflection, represented bylight beam 66, is caused by light reflected by the planar reflectivesurface that is parallel to the principle plane of the gain reflector,i.e., the display surfaces comprising, e.g., the front and rear surfacesof display medium 12 including any transparent overlay or covertherefor. With the above-described gain reflectors, an observer orobserving instrument 58 on viewing side 25 of the display not onlyreceives the highest gain of reflected light but also the highest glare.This occurs because the gain lobe 64 (reflected gain), comprisingreflected light beams 62, is distributed around the direction of thespecular reflection (glare), light beam 66, that is the angle ofincidence θ₁ equals the angle of reflection θ₂. However, if the viewer58 moves out of the glare angle, the available gain decreases.

For this reason, it is advantageous to utilize an offset gain reflector,one that separates the direction of the specular reflection or glarefrom the direction of the reflected gain or light.

The light distribution field or pattern from a display having an offsetgain reflector 14' is schematically illustrated in FIG. 5 where thereflected gain is represented by a light lobe 64', comprising lightbeams 62', and the specular reflection (the reflected glare) isrepresented by light beam 66'. As shown, the specular reflection orglare from the surfaces of display medium 12 is not in the samedirection as the maximum brightness and contrast of the display, i.e.,the specular reflection is angularly offset from light reflected (lightbeams 62') by the offset gain reflector.

The elimination of glare observable by an observer provides increasedoptical performance and further enhances the appearance of display 10.

The construction of a type of offset gain reflector 14' is schematicallyillustrated in FIGS. 6 and 7. The offset gain reflector illustrated is amodification of the gain reflector disclosed in the above-referenced'980 patent. More particularly, the optical elements of the offset gainreflector shown in FIGS. 6 and 7 comprise asymmetrical wave forms thatangularly offset the reflected light. As discussed, this modified gainreflector produces the light distribution pattern illustrated in FIG. 5.

The mathematical surface of the gain reflector of the '980 patent isbased on the joining (splining) together of individual low order curvedoptical elements in a manner which results in a shape with a continuousfirst derivative (tangent) and a defined second derivative (curvature).This is another way of saying that the optical elements are joinedtogether smoothly, with no sharp edges. This two dimensional (splined)wave form is then modulated in a non-standard fashion by another waveform defined on the orthogonal axis. The result is a three dimensionalsurface of smoothly joined optical elements having, as discussed, bothconvex and concave image-forming portions. The optical power (theability to spread light) of this array of elements is exactly the sameas the optical power of a single element.

The single optical element in the gain reflector of the '980 patent mayin principle be any smooth continuous function with two zero crossings;see, e.g., FIG. 6 where the "x"-"z" plane is the inflection or zeroplane and all "x" direction waveforms undergo zero crossing. But, inpractice, it has been restricted to second order functions, e.g., acircle, an ellipse, a parabola, or a hyperbola. The angle into which aray of light will be reflected by an optical element will depend on theslope of the curve at the point of interest, i.e., the steeper theslope, the larger the angle. Thus, the boundaries of the lightreflection pattern are determined by the steepest negative and positiveslopes.

An important consequence of the fact that the distribution of lightdepends only on the slope of a surface of an optical elements is thatthe mirror image of this surface has identically the same lightdistribution. It has the same focal length, but of opposite sign.

A second important consequence of slope dependence is that the lightdistribution pattern is element size independent. The smaller elementwill intercept and act upon a smaller portion of the incoming light, butwill distribute that light at the same angle or pattern as would alarger element. This is true whenever both the large and small elementshave the same shape.

The surface consisting of a single row of three dimensional opticalelements may be constructed by introducing some repeating wave form onthe axis orthogonal to the primary wave form. This is done by making theelement size proportional to the amplitude of the modulating wave. Thesurface of the gain reflector of the '980 patent is configured byjoining to each element its mirror image sized in such a manner that thecombined length of this compound element is held constant, independentof the size of the primary element. This pattern may be repeatedindefinitely creating a continuous sheet or array of optical elements inrows and columns as described in the '980 patent.

The gain reflectors produced commercially by Protolite have beensymmetric, and thus they produce symmetric light distribution fields orpatterns.

The offset gain reflector 14' schematically illustrated in FIGS. 5-7incorporates an asymmetric wave form element, which, as illustrated, maybe a section of a rotated ellipse. Other asymmetrical forms, such as arotated parabola, a rotated hyperbola or any high order curve, may beutilized to construct an offset gain reflector.

Specifically, the offset gain reflector 14' is made up of an array ofasymmetrical optical elements "A" each, e.g., comprising a section of arotated ellipse. As illustrated in FIG. 7, the result is a curve havinga much greater portion of its length with a positive slope "B" than anegative slope "C". Since the positive and negative slope regions sendreflected light to opposite sides, positive slope side "B", having morearea, will receive more light. Furthermore, the positive slope side hasless curvature and thus does not distribute light over as wide an angleas does the negative side "C". Since the distribution angle is smaller,the light remains more concentrated and thus brighter. The net result isan array or sheet of optical elements which, when light is directednormal to the elements, reflects that light in a distribution patternwhich is brighter on one side than the other.

FIG. 8 illustrates the reflected light pattern from the above-describedoffset gain reflector wherein the angle of reflection from 0° is plottedon the "x" axis and brightness on the "y" axis. As shown, the offsetgain reflector reflects incident light in a pattern that is brighter at"E", i.e., it is brighter on one side of the "y" axis than the other.

The offset gain reflector may be fabricated in the manner described inthe above-discussed SPIE paper by Mihalakis, i.e., by utilizing aComputer Numerical Control micro-milling system to form the asymmetricaloptical elements. Such an offset gain reflector can be manufactured byProtolite.

The optimum use of display 10 in quasi-collimated light depends to agreat degree on controlling the angle of the light incident on differentportions of display medium 12. Since in most applications, incidentlight is not completely collimated nor perpendicular to the displaysurface, different portions of display medium 12 and the gain reflector,for example the top and bottom surfaces, if the display is orientatedvertically, will receive light at different angles. This will, ingeneral, produce different optical performance and, therefore,noticeable differences in appearance.

However, in the embodiment illustrated in FIG. 9, the liquid crystalcell 12' and the gain reflector 14" are fabricated on thin flexibleplastic substrates, for example, that may be curved (in one dimension)so that more of the surface of the liquid crystal cell and the gainreflector are at the same angle of incidence (α₁ =α₂) relative to afront illumination source 80 (light from source 80 is represented bybeams 82,84) on viewing side 25 of display 10". In this embodiment,liquid crystal cell 12' most preferably comprises the encapsulatedoperationally nematic or NCAP liquid crystal material, and gainreflector 14" may be a sputtered aluminum coating, having an unevensurface, on the curved surface of the substrate. This configurationassists to ensure a uniform optical performance and appearance for thedisplay. It is especially suitable for use for vehicle dashboarddisplays.

The display apparatus of the present invention is operable in allambient lighting conditions to produce a display having excellentcontrast and brightness. The display is effective at night (very low,less than 100 foot lamberts ("Fl"), or zero ambient light), in brightsun (ambient light greater than 1000 Fl), and on cloudy days or indoors(ambient light 100 to 1000 Fl).

As discussed, incident light, represented by light beam 60 (see FIGS.2,5), is transmitted through the display medium, in the non-scatteringstate, where it is reflected, shown as light beams 62, by means of thegain reflector to create a display observable by an observer 58.

As noted, dye in the NCAP liquid crystal cell produces light absorption.For incident light that is quasi-collimated, the brightness of thedisplay in the field-on state is increased because the light experienceslittle absorption or scattering as it passes through the liquid crystalcell. It is then reflected into a narrow distribution by the gainreflector and then passes through the liquid crystal cell again.

In the field-off state, incident light is strongly scattered andabsorbed. Thus, the portion of the light that reaches the gain reflectoris much more diffuse than the incident beam. Therefore the effectivegain of the gain reflector will be much lower. Therefore, the displaywill be brighter and have a higher contrast.

When incident light is diffuse, the brightness in the field-on state isstill increased by the gain reflector. This is caused by thedifferential absorption of light that enters the liquid crystal cellfrom different angles.

For instance, consider a light ray, represented by beam 72 (see FIGS. 1,3-4) that is normal to the liquid crystal cell structure. Since, in thefield-on state, the NCAP liquid crystal material is aligned by theelectric field E there will be little scattering or absorption for lightthat is normal to the liquid crystal cell, such as light ray 72. Forlight rays at an angle from the normal, for example 45°, there will besignificant absorption since these rays will not be traveling parallelto the field aligned liquid crystal 40 and the dye 42. Since light ismore strongly scattered and absorbed the further its incident directionis from the normal to the surface of the liquid crystal cell, thetransmitted light distribution is narrowed somewhat from the incidentdistribution. This narrow distribution will be reflected by the gainreflector with some gain.

In the field-off state, the liquid crystal cell will scatter and absorbthe diffuse incident light and provide little or no gain. Therefore,even in diffuse lighting conditions, the combination of the gainreflector and the liquid crystal cell provide high brightness andcontrast.

The display apparatus of the present invention, for the various modes ofoperation, displays the selected numeral, character or other informationto an observer 58 on viewing side 25 within the viewing angle of thedisplay. For example, to observer 58, the area between the energizedelectrodes may appear very light (or colored) against a very darkbackground. The liquid crystal material that is not located between theenergized electrodes is in the field-off state. Thus, that materialstill scatters and absorbs incident light, creating a very darkappearance, from both the viewing and non-viewing sides.

The display of the present invention is adaptable to such displays asvehicle dashboards and control panels.

The display of the present invention produces unique displayimprovements including: (1) the overall brightness of the display whenthe display medium is in the non-scattering state is increased while thedisplay brightness in the scattering state is not changed, yielding anincrease in the contrast ratio when the display is illuminated by eithercollimated or quasi-collimated light; (2) an improvement in thebrightness and contrast ratio of the display even when illuminated bydiffuse light; and (3) the capability of angularly separating thereflected light distribution (reflected gain from the gain reflector)from the specular reflection (glare from the display medium in front ofthe gain reflector).

Although certain specific embodiments of the invention have beendescribed herein in detail, the invention is not to be limited only tosuch embodiments, but rather only by the appended claims.

What is claimed is:
 1. A display apparatus comprising:a liquid crystal means at a viewing side of the display; said liquid crystal means comprising a liquid crystal material in a containment medium means for inducing a distorted alignment of said liquid crystal material which in response to such alignment at least one of scatters and absorbs light and which in response to a prescribed input reduces the amount of such scattering and absorption; and a gain reflector means disposed behind said liquid crystal means for reflecting light passing through said liquid crystal means wherein the contrast ratio of the display is increased by the gain of said gain reflector.
 2. A display apparatus comprising:a liquid crystal means disposed at a viewing side of the display; said liquid crystal means comprising operationally nematic liquid crystal containing a dye and a containment medium means for containing plural volumes of the liquid crystal and dye, said containment medium means having surface means for distorting the natural structure of the liquid crystal to cause the dye to increase light absorption, and the liquid crystal being responsive to a prescribed input to reduce the amount of such light absorption; and gain reflector means for reflecting light incident thereon, said gain reflector means disposed at a non-viewing side of the display behind said liquid crystal means wherein the contrast ratio of the display is increased by the gain of said gain reflector.
 3. The apparatus of claim 2 wherein said gain reflector means provides that specular reflection from the display is angularly offset from the reflected gain.
 4. The apparatus of claim 3 further including a color filter means disposed between said liquid crystal means and said gain reflector means.
 5. The apparatus of claim 2 wherein said gain reflector means includes a pattern of fluorescent colors.
 6. The apparatus of claim 2 wherein said gain reflector means comprises a plurality of similar units disposed in substantially contiguous relation, each unit including an emitting surface which is convex in a plane defined by first and second axes and concave in planes perpendicular thereto.
 7. The apparatus of claim 2 wherein said gain reflector means comprises a roughened mirrored surface.
 8. The apparatus of claim 2 wherein said liquid crystal means has electrode means formed at the opposite surfaces thereof for applying an electric field across said liquid crystal as the prescribed input.
 9. The apparatus of claim 8 further comprising circuit means for providing electric energy to said electrode means to effect application of the electric field to said liquid crystal.
 10. The apparatus of claim 2 wherein said liquid crystal means and said gain reflector means present a curved surface to an illumination source on a viewing side of the display.
 11. The apparatus of claim 10 utilized as part of a vehicle dashboard and further including a front lighting source.
 12. The apparatus of claim 2 utilized in a vehicle dashboard.
 13. The apparatus of claim 1 wherein said liquid crystal is a high birefringent material and said dye is a pleochroic dye.
 14. The apparatus of claim 1 wherein said gain reflector means comprises a plurality of similar units disposed in substantially contiguous relation, each unit including an emitting surface which is concave in a plane defined by first and second axes and convex in planes perpendicular thereto. 